Ergothioneine is a naturally-occurring amino acid and is a thiourea derivative of histidine, containing a sulfur atom in the imidazole ring. This compound is made in rather few organisms, notably Actinobacteria and filamentous fungi.[1] Ergothioneine was discovered in 1909 and named after the ergot fungus from which it was first purified, with its structure being determined later, in 1911.[2] This amino acid has antioxidant properties, but its chemistry differs from conventional sulfur-containing antioxidants such as glutathione or lipoic acid. Although ergothioneine cannot be made in human cells, it is present in some tissues at high levels as it is absorbed from the diet.[3] In humans ergothioneine is taken up from the gut and concentrated in the tissues by a specific transporter called the novel organic cation transporter. However, even today, one hundred years after its discovery, precisely what ergothioneine does in the human body remains a mystery